[dm-crypt] Alignment issue with 4K disk

Sven Eschenberg sven at whgl.uni-frankfurt.de
Mon Jan 4 23:41:11 CET 2016


The Startsector of the Partition should NEVER be odd, rather even. 65536 
would be fine, that's why lsblk gives an alignment of -1 for 
scapa-backup I guess.

It does not matter if you use MBR or GPT, well, not too much for this 
matter. The GPT is 34 LBA blocks (of 512 bytes). It can possibly be 
longer to host more than 128 Partition entries, but no way close to 2048 
which would resemble 1MByte. So 2048s would be a good place to start 
your partition. It is aligned to 1 MByte, as well as 4KByte ... it even 
is aligned to all major SSD Page-Sizes and SSD Erase-Block-Sizes.

Anyhow you either need to align the start of your partition or you'll 
have an extremly hard time aligning the dm-crypt offset manually at the 
right place.

Regards

-Sven

P.S.: If you still have a chance to change things, better do it now!


Am 04.01.2016 um 23:20 schrieb Yves-Alexis Perez:
> On lun., 2016-01-04 at 22:54 +0100, Sven Eschenberg wrote:
>> Hi,
>>
>> start=4194304 -> assumind this is in 512 byte blocks, the start sector
>> is aligned to MByte boundary and thus 4k boundary.
>>
>> Anyway, if your device only has one partition which is encrypted with
>> dm-crypt, set it up to start at sector 2048 (which should be default for
>> parted, fdisk and so on). Pass align-payload 2048 during creation of the
>> LUKS header, which will give you an MByte boundary and should always be
>> okay.
>>
>> If the kernel still complains, then something else might be going wrong.
>
> Right now I have:
>
> root at scapa:~# lsblk -o +ALIGNMENT /dev/sdc
> NAME             MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT        ALIGNMENT
> sdc                8:32   0 931.5G  0 disk                            0
> └─sdc1             8:33   0 931.5G  0 part                          512
>    └─scapa-backup 253:4    0 931.4G  0 crypt /mnt/scapa-backup        -1
>
> And:
>
> scapa-backup: 0 1953336209 crypt aes-xts-plain64
> 0000000000000000000000000000000000000000000000000000000000000000 0 8:33 65536
>
> root at scapa:~# fdisk -l /dev/sdc
> Disk /dev/sdc: 931.5 GiB, 1000204886016 bytes, 1953525168 sectors
> Units: sectors of 1 * 512 = 512 bytes
> Sector size (logical/physical): 512 bytes / 4096 bytes
> I/O size (minimum/optimal): 4096 bytes / 33553920 bytes
> Disklabel type: gpt
> Disk identifier: EBA98857-89BA-4B2C-8366-4A77A527A8E0
>
> Device     Start        End    Sectors   Size Type
> /dev/sdc1  65535 1953467279 1953401745 931.5G Linux filesystem
>
> Partition 1 does not start on physical sector boundary.
>
> [I'm a bit concerned with that, but since I'm using a GPT and not MBR it might
> be related]
>
> root at scapa:~# gdisk -l /dev/sdc
> GPT fdisk (gdisk) version 1.0.1
>
> Partition table scan:
>    MBR: protective
>    BSD: not present
>    APM: not present
>    GPT: present
>
> Found valid GPT with protective MBR; using GPT.
> Disk /dev/sdc: 1953525168 sectors, 931.5 GiB
> Logical sector size: 512 bytes
> Disk identifier (GUID): EBA98857-89BA-4B2C-8366-4A77A527A8E0
> Partition table holds up to 128 entries
> First usable sector is 34, last usable sector is 1953525134
> Partitions will be aligned on 8-sector boundaries
> Total free space is 123356 sectors (60.2 MiB)
>
> Number  Start (sector)    End (sector)  Size       Code  Name
>     1           65535      1953467279   931.5 GiB   8300  scapa-backup
>
> Regards,
>
>
>
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